1. Field of the Invention
The present invention relates to a print apparatus that forms an image by printing a print agent on a print medium.
2. Description of the Related Art
OA equipment such as personal computers and word processors has recently been prevailing widely. Accordingly, various print apparatuses have been provided which print information output by these pieces of equipment on a variety of print media. In particular, print apparatuses based on an ink jet system have various advantages such as low noise, low running costs, small sizes, and the ease of introduction of colors. These print apparatuses are thus accepted by a wide variety of users. There has recently been a growing demand for improvement of images picked up using digital cameras; the grade of output images is desirably similar to that of silver salt photographs. In response to this, improved printing methods are now carried out. For example, owing to the configuration of print apparatuses, the accuracy with which print media are conveyed tends to decrease at the leading or trailing end portion of the print media. Thus, ink jet print apparatuses have already been provided which employ a special printing method for the leading or trailing end portion of print media. Further, print apparatuses are now common which adopt a printing method called “multi-pass printing” in order to output smoother images having a quality equivalent to that of photographs. A brief description will be given of the specific configurations of the “leading and trailing end portion printing” and “multi-pass printing”.
(Method for Printing Leading and Trailing End Portions)
When the leading or trailing end portion of a print medium is printed, the print medium may depart from part of a plurality of rollers that convey the print medium while supporting it in its front and rear. As a result, the image may be disturbed. This will be described below in detail.
FIG. 1 schematically shows a print head, a print medium, and a conveying mechanism that conveys the print medium while supporting it. The figure shows that the middle portion of the print medium is being printed. In the figure, three nip portions are formed by a conveying roller M3060, two sheet discharging rollers M3100 and M3110, and spurs M3070 and M3120 located opposite these rollers. The rollers and spurs spread and support the print medium. Rotation of the three rollers enables the print medium P to be conveyed in a direction shown by the arrow in the figure.
Reference numeral H1000 denotes a head cartridge. The head cartridge has a plurality of printing elements arranged at a predetermined pitch in the conveying direction in the figure to eject ink. The head cartridge H1000 causes the printing elements to eject ink while moving and scanning in a direction in which the head cartridge moves away from the reader. The head cartridge thus forms an image in an area of the print medium P located between the conveying roller M3060 and the auxiliary roller M3100. Images are sequentially formed on the print medium P by alternately repeating a printing scan by the head cartridge H1000 and an operation performed by the roller pair to convey the print medium.
FIG. 2 shows that printing progressed compared to the state shown in FIG. 1 and that the vicinity of the trailing end portion of the print medium P is being printed. The print medium P has already departed from the conveying roller M3060. The print medium is being conveyed only by rotation of the sheet discharging rollers M3100 and M3110.
In general, the conveying roller M3060 and the sheet discharging rollers M3100 and M3110 often have slightly different roller diameters and conveyance accuracies owing to their major roles. The major role of the conveying roller M3060 is to position the print medium at an appropriate position for the print head for each printing scan. Thus, the conveying roller M3060 has a sufficient roller diameter and can perform a conveying operation with a desired accuracy. In contrast, the major roll of the sheet discharging rollers M3100 and M3110 is to reliably discharge the printed print medium. The sheet discharging rollers M3100 and M3110 often have smaller roller diameters and a lower print media conveying accuracy than the conveying roller M3060. That is, the conveying accuracy may decrease in an area corresponding to the period after the trailing end portion of the print medium P departs from the conveying roller M3060 and before the trailing end portion of the print medium P is completely printed, compared to the preceding area. In this case, depending on a printed image, black stripes may be observed if conveyance amount is insufficient or white stripes may be observed if the conveyance amount is too large. This may adversely affect the image.
Moreover, the image is also adversely affected by the lack of retention of the opposite ends of the print medium. When the trailing end portion of the print medium P departs from the conveying roller M3060, the distance between the print head and print medium (this distance will be referred to as a head-medium distance below) varies greatly and subsequently becomes unstable. An inkjet print head moves and scans while ejecting ink at a time corresponding to a predetermined head-medium distance maintained by the front and rear rollers. Ink ejected at an appropriate time forms dots on the print medium. The dots are arranged at an appropriate pitch to form an image. Accordingly, if the head-medium distance is changed during printing or subsequently varies significantly, the positions of dots on the print medium become unstable. As a result, a defect may occur such as white or black stripes or the sense of roughness. This head-medium distance problem occurs not only at the trailing end portion but also the leading end portion of the print medium during printing.
FIG. 3 shows that the vicinity of the leading end portion of the print medium P is being printed. The print medium P is being held and conveyed only by the conveying roller 3060. For printing of the leading end portion, the print medium P is not conveyed by the sheet discharging rollers M3100 and M3110 but by the conveying roller M3060. Accordingly, the conveyance is carried out more precisely than in the case in which the vicinity of the trailing end portion is printed as described in FIG. 2. However, the head-medium distance problem, caused by the lack of retention of the leading end portion of the print medium P, also occurs in this case as shown in FIG. 2. That is, the accuracy of dot positions on the print medium is more unstable than that in the printing of middle portion of the print medium.
To deal with the image problem associated with the leading and trailing end portions of the print medium, serial ink jet print apparatuses, which attach great importance particularly to image grade, commonly adopt the following method. The printing width (that is, the number of printing elements that actually eject ink) of the print head is reduced only at the leading and trailing end portions to correspondingly decrease the amount of print medium conveyed (see, for example, Japanese Patent Application Laid-Open No. 2004-98668). The reduction in the printing width of the print head makes it possible to suppress a variation in head-medium distance with respect to the printing width. This is effective particularly on the sense of roughness, which may occur when multi-pass printing is carried out as described later. Further, even with a decrease in conveyance accuracy, the resulting conveyance error can be reduced by lessening the print medium conveyance amount. Moreover, the pitch decreases in the intervals between print areas to make white or black stripes unmarked.
Ink jet print apparatuses employing an interlace printing method also adopt the method of reducing the number of printing elements actually ejecting ink, only at the trailing end portion to correspondingly decrease the print medium conveyance amount; these ink jet print apparatuses use a print head in which printing elements have an arrangement density lower than a recording density to complete an image while interpolating printing density in the sub-scanning direction of printing scan (see, for example, Japanese Patent Application Laid-Open No. 11-291506 (1999)).
(Multi-pass Printing Method)
The multi-pass printing method will be described below in brief.
FIG. 4 schematically shows a print head and a print pattern. Reference numeral P0001 denotes a print head. In this case, the print head is assumed to have 16 printing elements for simplification. As shown in the figure, the printing elements are divided into four printing element groups, a first to fourth groups. Each printing element group includes four printing elements. Reference numeral P0002 denotes a mask pattern, areas printed by the printing elements are painted with black. The patterns printed by the respective printing element groups are complementary to one another. Superimposing these patterns completes printing the 4×4 area.
The patterns shown by P0003 to P0006, show how the image is completed by repeating printing scans. Every time a printing scan is finished, the print medium is conveyed by a distance corresponding to the width of each printing element group in the direction of the arrow in the figure. Thus, the image in the same area (corresponding to the width of each printing element group) in the print medium is completed by four printing scans.
As described above, each area of the print medium is formed by using the plurality of printing element groups to execute a plurality of scans. This is effective on reducing a variation intrinsic to the printing elements, a variation in print medium conveyance accuracy, or the like. Further, modifying the arrangement of the mask patterns also makes it possible to deal with various other image problems, the reliability of the print apparatus.
For example, with the recent ink jet print head ejecting a large number of small droplets at a high frequency, the direction in which ink is ejected from the printing elements located at an end of the print head may be skewed inwardly. In this case, dots formed by the printing elements located at the end of the printing element row impact a position inward of the regular one. As a result, white stripes (referred to as an end-deviation below) appear at a pitch equal to the printing width of the print head. Even in this situation, the modification of the arrangement of the mask patterns can make the end-deviation unmarked (see, for example, Japanese Patent Application Laid-Open No. 2002-096455).
FIG. 5 is a diagram showing an example of mask patterns employed to make the end-deviation unmarked. The black areas in the mask patterns in FIG. 5 play the same role as that of the black areas in the mask patterns in FIG. 4. The black areas permit printing. The white areas in the mask patterns in FIG. 5 play the same role as that of the white areas in the mask patterns in FIG. 4. The white areas do not permit printing. In this example, a print head having 768 printing elements is used to carry out 4-pass printing. As in the case of FIG. 4, the 768 printing elements are divided into four printing element groups. In this case, printing rate (the percentage for which the number of black areas accounts in the total number of black and white areas constituting the mask patterns) varies depending on the positions of the printing elements. The printing rate of the mask pattern corresponding to the first printing element group is N. The printing rate of the mask pattern corresponding to the second printing element group is M (M>N). The printing rate of the mask pattern corresponding to the third printing element group is M. The printing rate of the mask pattern corresponding to the fourth printing element group is N. The sum (N+M+M+N) of the printing rates of the mask patterns corresponding to the four printing element groups is 100%. In this manner, the printing rate of the printing elements in the middle portion is set at a relatively large value but decreases gradually toward the end of the row of the printing elements. The above inward skewing of the ejecting direction has been found to be significant as smaller ink droplets are printed more densely and faster. Accordingly, setting the printing rate of the end of the row of the printing elements lower than that of the middle portion, it is possible to weaken the tendency to inwardly skew of the printing elements located at the end. Further, even with the presence of this tendency, an end-deviation resulting from deviations in printing positions is made insignificant because of a decrease in the number of dots ejected from the printing elements located at the end.
For ink jet print apparatuses focusing on photographic image quality, important elements are a decrease in the volume of each droplet, an increase in the density of printing elements, and an increase in driving frequency. Thus, such mask patterns as shown in FIG. 5 are generally useful for the recent ink jet print apparatuses attaching great importance to photographic image quality.
As already described, print apparatuses focusing particularly on image quality reduce the number of printing elements to use at the leading and trailing end portions. Thus, to simultaneously carry out such leading and trailing end portion printing and multi-pass printing, the mask patterns for the multi-pas printing must be switched at the leading and trailing end portions of the print medium. Such a configuration is already disclosed in Japanese Patent Application Laid-Open No. 2002-144637.
In many cases, measures for suppressing the adverse effect of the end-deviation are often required for the leading and trailing end portions at which printing is carried out with a reduced number of printing elements. Accordingly, even in the mask pattern applied to the leading or trailing end portion, the printing rate of the terminal of this end is commonly equivalent to that of the terminal at printing for the middle portion of the print medium regardless of the number of printing elements used for printing.
However, with the conventional printing method, the number of printing elements used to print the leading or trailing end portion is sharply reduced compared to that of printing elements used to print the middle portion. This disadvantageously increases the time required to print the leading and trailing end portions. That is, even if the print apparatus which has a large number of printing elements and is capable of printing at high speed, the printing speed is lower when the leading or trailing end portion is printed. As a result, the print apparatus cannot fully exhibit its high speed performance. Under these circumstances, the inventors concentrated their energy on examinations to find that the adverse effect on an image at its leading and trailing end portions can be suppressed to some degree by modifying applied mask patterns. The inventors thus believe that by applying such mask patterns to the areas of the leading and trailing end portions, it is possible to output images at almost the same level as that achieved with the conventional technique, at a speed higher than that achieved with the conventional technique, without the need to reduce the number of printing elements used for printing down to the conventional level.